Hi everyone, I am having a slight problem with my shower. Sometimes I will hear a popping/banging noise and the water will either turn completely hot or completely cold. The shower is over 20 years old, and it has the two individual handles.

Is there a mixing valve in this type of system? I am trying to figure out what needs to be replaced, and then how to do it. Any help is appreciated. Thanks HRT!

Welcome Shoobie:
Popping and banging in water pipes usually comes from air trapped in the systme. A suspect soruce of the air is the water heater, especially if it is gas. Scale builds up in the bottom of the tank and when the gas comes on you can hear the air bubbles coming up through the scale; it will make a rumbling noise.
When the air bubbles hit the shower valve they prevent one or the other sides from coming through, thus the instant change in temperature. You could put an air vent in a high point of the hot water pipe over the water heater. That will expell the air from the system at the source.
Glenn

No, you do not have a 'mixing valve' - You mixing is manual by adjusting the hot and cold independently.

What Glenn is describing is called "water hammer". It could very well be the cause.

Let me give you a simple thing to check first, just to rule out a possible different issue. You could possibly just have a bad washer screw and washer. Its a 10 cent fix and one trip to the hardware store (hopefully).

Shut off the water. Remove the handle and trim from the fixture. You will see a large nut, called the packing nut. Remove it - Lefty loosey. Now use the handle you removed to unscrew the valve stem (the part where the handle attaches). It turns in the direction you turn the water on. Remove the stem. At the end of the stem should be a small brass or copper screw holding a washer. My guess is that the head of he screw is gone and the washer is flopping around. Replace the washer and the screw. Reverse the procedure to re-assemble. If you have a drip where the packing nut meets the stem you may also need some new packing material (another buck). Maybe pick some up while you are getting your washers and screws.

I'm having the exact same problem with my hot water shutting off. I also have two separate valves. I haven't heard a clunk, it just diminishes hot water flow fairly quickly, with or without the cold water valve open.

The problem started some time ago, but the problem was minimal. I just replaced the cold water valve stem due to a bad valve seat, and now the hot water shuts off completely. The valves are old, I had to go to a specialty shop to find the right replacement. When I put in the new cold valve, I replaced the washer on the hot water valve and the screw appeared to be solidly in place. I didn't see any packing - is it possible my style valve doesn't have it?

Lilly - The packing is inside/under the nut that you removed to get the valve stem out. It prevents leaks from between the stem and the nut. It wont contribute to an issue of diminishing hot water.

Check to make sure the washer screw didn't come loose on the hot side. While you have it off look inside the valve body and make sure the seat where the washer contacts is clear. Many of those seats are replaceable with a special tool. The seat may be loose. A small screwdriver pokeing around will allow you to know if you have loose seat.